The present invention generally relates to a combustion apparatus and more particularly, to a catalytic combustion arrangement for use in a gas heater and the like or other industrial combustion equipment in general.
In a conventional catalytic burner, it has been the practice, for example as shown in FIG. 1, that a heat-resistant wire netting or wire mesh W, a catalytic mat M supporting a catalyst, a diffusion heat insulation layer DH, and a supporting plate S having through-holes h formed therein, are piled one upon another in that order from above into one unit constituting a catalytic mat layer L which is accommodated in a casing H, with the entire peripheral edge of said heat-resistant wire mesh W being depressed from above by a cover member C for fixing. In the known construction as described above, however, since the catalytic mate M directly contacts a flange portion Ha of the casing H, heat at the contact portion of the mat M is absorbed by the flange portion Ha, with a consequent reduction of combustion rate or combustion and hydrocarbon emission rate (which may be represented by: volume of hydrocarbon discharged from the mat M/total volume of hydrocarbon supplied to a combustion apparatus) during a steady combustion at said portion.
Meanwhile, in a known igniter construction for the catalytic mat M as shown in FIG. 2, upon opening of a cock K, gas is fed through a gas pipe P to the mat M through a gas nozzle N, while simultaneously, a pilot flame burner F is ignited, and thus, the fuel within the mat M is gradually ignited from the portion surrounding the pilot burner F for combustion throughout the mat M as a whole. However, in the conventional arrangement as described above, the combustion rate at the early stage of combustion is extremely low, since the catalytic combustion of the fuel is started from the portion surrounding the preheating burner, and unburnt gas is discharged from other portions which have not reached the temperature of initiation of catalytic combustion.
Alternatively, in another conventional catalytic mat arrangement as shown in FIG. 3, as electric heater element E is embedded within the catalytic mat M, and the heater element E is energized in association with opening of the cock K and is kept energized during combustion, or is de-energized upon arrival at the steady combustion state. In the prior art arrangement as described above, however, an electric circuit including the electric heater element is required in addition to the gas line, and thus, the construction is undesirably complicated, with a consequent rise in cost.